Molded thermoplastic materials, including polyolefins, are used in a variety of applications, such as the transportation, automotive, marine, recreation, construction, office products, and lawn and garden equipment manufacturing industries. Their use, however, is not without problems. In many instances, molded thermoplastic work pieces may need to be coated to facilitate paint adhesion, or to satisfy other surface property requirements, such as durability and weather resistance. Because of the inherent low surface energy of thermoplastics, generally, and in particular, polyolefins, they are difficult to paint or coat. Moreover, in view of the variation among the surface properties of individual polyolefins and the coating compositions to be applied, a method that works with one specific thermoplastic may not work with another. Hence, a variety of methods have been developed to achieve adhesion of coatings to the surface of molded thermoplastics materials, including materials such as polyolefins.
One of the most common methods is to micro-etch the surface of the thermoplastic to generate micro-roughness that will provide adhesion-anchoring sites for the paint or other top and primer coatings. Etching may be done by solvents, which may be incorporated in the paint or coating being applied. The selection of solvent is critical because different solvents etch thermoplastics at different rates. Both over-etching and under-etching must be avoided. Insufficient etching does not provide proper adhesion; excessive etching can damage the thermoplastic. Excessive etching, exposing the coating to bleeding from the thermoplastic, or exposing the thermoplastic to attack by the solvent may warp thermoplastic parts. If thermoplastics have areas that are highly stressed by the molding process, use of etching solvents can form visible cracks in these areas.
Another method of preparing the surface of a thermoplastic part for painting or coating is through de-glazing. When some thermoplastics are molded, a highly crosslinked (glazed) skin is formed which is resistant to solvent etching. Tumbling with a moderately abrasive media, or blasting with a mildly aggressive grit material, may de-glaze the thermoplastic surface sufficient to allow satisfactory adhesion of the paint or coating.
Creating micro-roughness through etching or de-glazing may not be desirable and, in some instances, not effective, depending on the particular thermoplastic surface involved. Other methods to prepare a thermoplastic surface utilize a chemical reaction to create polar oxidized groups on the thermoplastic surface. These surface polarizing methods include coating with an adhesion promoter, or subjecting the polyolefin work piece to flame or plasma treatment, in order to make the thermoplastic surface chemically polar so it will bond with the coating. Low polarity thermoplastics can also be oxidatively surface treated using light sensitive chemicals called photosensitizers, followed by exposure to ultraviolet light. UV light cracks the molecules of the photosensitizers for form free radicals. Free radicals are extremely reactive species that combine with oxygen in the air. Oxygen free radicals, in turn, react with the thermoplastic to produce polar groups on the thermoplastic surface.
Previously, molded thermoplastic work pieces were formed in a mold, the molded product removed, and a coating was then applied on the surface of the molded work piece by a coating process, such as a surface treatment, primer coating, top coating, painting, etc. Hence, all of the foregoing methods required an additional step to achieve a finished surface on a thermoplastic work piece, which is treating the surface of the pre-formed thermoplastic work piece prior to applying a paint or coating. These methods required additional steps and increased costs of preparing the molded work piece surface.
It became desirable, therefore, to have a method by which a coating could be applied to a thermoplastic work piece in the mold, resulting in a coated thermoplastic work piece the surface of which would be finished and suitable for use as is in an end use application, or which would require less surface preparation treatment than heretofore utilized.
Application of in-mold coatings (IMC) to thermoplastic materials to provide generally smooth surfaces, improve durability and other surface properties, and to reduce or eliminate substrate porosity is known. A number of in-mold coating methods have been employed for applying primer coatings, in compression molding methods or injection molding methods employing molding materials of thermosetting resins, such as SMC (sheet molding compound) and BMC (bulk molding compound) (e.g., U.S. Pat. Nos. 4,076,788; 4,081,578; 4,331,735; 4,366,109; and 4,668,460).
Typical in-mold coatings are set forth in U.S. Pat. No. 4,189,517 and U.S. Pat. No. 4,222,929, which have been applied to fiber reinforced thermoplastics (FRP), such as sheet molding compounds, and which are the reaction products of an unsaturated fumarate polyester diol, a saturated polyester diol flexibilizer, a crosslinking aliphatic polyol, having from 3 to 6 hydroxyl groups, a diisocyanate, and an ethylenically unsaturated crosslinking compound, such as styrene. U.S. Pat. No. 4,331,735 sets forth a liquid crosslinkable composition having an average molecular weight of up to about 5,000 and a plurality of polymerizable ethylenic double bonds, being essentially free of active hydrogen atoms or being essentially free of isocyanate groups; a material such as a polyisocyanate or a reaction product of a polyisocyanate and an ethylenically unsaturated compound having —NH2 groups, —NH and/or —OH groups, said reaction product being free of active hydrogen atoms; and an organic free radical peroxide initiator.
Other coatings relate to those comprising at least one polymerizable epoxy-based oligomer having two acrylate groups thereon, at least one copolymerizable ethylenically unsaturated monomer such as styrene, and at least one copolymerizable monoethylenically unsaturated compound having a —CO— group and a —NH2, —NH—, and/or —OH group, as well as polyvinyl acetate, as set forth in U.S. Pat. Nos. 4,414,173 and 4,515,710 to Cobbledick et al.
Still other coatings include a conductive, thermoset in-mold coating for molded FRP parts, the binder of which comprises at least one polymerizabie epoxy-based oligomer having at least two acrylate groups and at least one copolymerizable ethylenically unsaturated monomer, which provides good flow and coverage during molding, good adhesion, uniform color, good surface quality, and good paintability, as set forth in U.S. Pat. No. 5,614,581. Still other in-mold coatings include free radical peroxide initiated thermosetting compositions comprising an epoxy-based oligomer having at least two acrylate end groups and a hydroxy or amide-containing monomer, as set forth in U.S. Pat. Nos. 5,391,399; 5,359,002; and 5,084,353 to Cobbledick et al.
In-mold coating compositions, which have appearance or paint-like properties, are also known. Appearance in-mold coating compositions are desirable because they eliminate the additional step, time and cost of applying paint to the surface of an in-mold coated work piece.
One such appearance in-mold coating is disclosed in U.S. Pat. No. 5,736,090. The '090 patent relates to a method of coating a polyamide work piece by utilizing an in-mold coating composition capable of providing a coating having sufficient durability with respect to adhesion, appearance, and weather resistance, and which functions as a top coating applicable to exterior parts of automobiles or other outdoor applications. The in-molding coating composition comprises, as a vehicle component, a urethane acrylate oligomer or a urethane methacrylate oligomer and a polymerizable unsaturated monomer; a polyisocyanate compound; and a polymerization initiator, where the oligomer itself is a reaction product of an organic polyisocyanate, an organic polyol, and a hydroxy alkyl acrylate and a hydroxy alkyl methacrylate.
Another example of an appearance in-mold-coating is the cured in-mold coating composition suitable for use on fiber reinforced thermoplastic (FRP), which comprises a saturated polyester urethane acrylate made from a saturated aliphatic polyester intermediate, a saturated aliphatic urethane group and a saturated hydroxyl (alkyl) (meth) acrylate, as set forth in U.S. Pat. No. 5,777,053, the disclosure of which is incorporated herein by reference. The '053 patent relates to the use of a diacrylate ester of an alkylene diol, a saturated (cyclo) aliphatic (meth) acrylate, and a vinyl substituted aromatic to impart paint coating type properties to the in-mold coating composition, such as hardness, water resistance, low shrinkage, and high gloss. Optionally, and in addition to the aforenoted components, crosslinking agents, such as triallylcyanurate, ethoxylated trimethylolpropane triacrylate, pentaerythritol triacrylate and the like may be utilized. The components are reacted in the presence of an initiator, such as a peroxide, to chain extend and to form the thermoset saturated polyester urethane acrylate coating resin. The cured resin is a clear in-mold coating composition, which, if desired, may be pigmented using various pigments, colorants, etc., known to the art, to yield a desired end color and opacity. Appearance or paint-like properties of these in-mold coatings are achieved by avoiding various components, especially aromatic compounds such as aromatic polyesters and/or polyether urethane intermediates, aromatic epoxy-based resins and the like. These compositions have been used successfully to form a paint-free FRP end product laminate. The FRP molds were prepared in a closed mold from polyester SMC. Molding conditions for the SMC were 300° F. (149° C.), a 70 second cure time, and 1000 psi of pressure. The in-mold coating compositions were applied immediately following SMC cure by opening the mold, injection or otherwise applying the coating onto the FRP molding, followed by re-closing of the mold. The cure conditions for the IMC were 300° F. (149° C.), a 60 second cure time, and 1000 psi of pressure.
In view of the predominance of the use of injection molded polyolefin substrates in the transportation, automotive, marine, recreation, construction, office supply, and lawn and garden manufacturing industries, it is desirable to provide an in-mold coating method for use in injection molding of polyolefin work pieces. Due to the above-described, inherently low surface energy of polyolefins, which creates coating adhesion issues, and the lower, standard molding temperatures (150-170° F.) utilized in injection molding of polyolefins, insufficient covering or adherence to the polyolefin work piece by the in-mold coating has been difficult to achieve.
One method for coating a polyolefin work piece in a way, which avoids having to apply an additional coating of paint to a preformed part, is disclosed in U.S. Pat. No. 5,562,979. The '979 patent relates to a dual injection molding technique which involves heating a powdered plastic paint coating material to its plastic phase and then injecting it under pressure into a mold, followed by injecting a thermoplastic substrate material under pressure into the mold to cause the coating material to coat a surface of the mold, thus producing a work piece coated by the plastic paint coating material. The paint and substrate materials are selected so as to have an affinity for each other, and the method may include effecting cross-linking between the coating and substrate during molding and curing. The method is illustrated using a polypropylene substrate heated to a temperature of 230° C. (446° F.) to enable it to be extruded into the mold at a pressure of 1300 bar. The method is disadvantageous, however, because it requires additional time to grind the paint material, which is normally produced as a solid sheet, into a powder or into a granulated form, and to heat the ground or granulated material to its plastic phase. Another disadvantage of this method is that it requires the use of two separate extruders. Still another disadvantage of the method, which is very limiting, is that requires that the materials selected have affinity for each other, or that the selected materials be chemically modified to work together.
It has been discovered that injection molding of polyolefin substrates and coating with the in-mold coating compositions, as described in the '053 patent above, was successful in making coated polyolefin parts having a thoroughly cured coating. Furthermore, the coating exhibited good adhesion to the substrate. For purposes of the present invention, the use of a free radical initiator, as a chain extension component, in conjunction with the curing monomers of the described in-mold coatings, is thought to be important to the quality of the appearance and the properties obtained. While not wishing to be bound by any theory, it is believed that the use of a free radical initiator, such as a peroxide compound, promotes the adhesion of the in-mold coating composition to the surface of the polyolefin work piece. It is thought that the free radicals generated within the coating composition react with the surface of the polyolefin in some manner and thereby permit a bonding or adhesion of the coating to the polyolefin.
A process by which polyolefin substrates having in-molded coatings thereon leas been developed. In-mold coating of polyolefin work pieces, whereby the coating composition has good flow and coverage during molding, good adhesion, uniform color, good surface quality, and, if necessary, good paintability, may be successfully achieved during injection molding processes, by increasing only slightly the temperature at which the polyolefin substrate is injection molded and through the use of the above-described, standard in-mold coatings, comprising a free radical initiator, such as a peroxide compound.
It is an object of the present invention to provide an injection molding process by which thermoplastic substrates may be coated with in-mold compositions, to form finished work pieces which are suitable for use as is in an end use application or which require minimal surface post-treatment.
It is an object of the present invention to provide an injection molding process by which polyolefin substrates may be coated with in-mold compositions, to form finished polyolefin work pieces which are suitable for use as is in an end use application or which require minimal surface post-treatment.
It is a further object of the present invention to eliminate the time and cost of pretreating a pre-formed thermoplastic or polyolefin work piece to accept a paint or other coatings thereon.
A further object of the present invention is to eliminate the need of applying additional paint or other surface treatment coatings to a surface of a pre-formed thermoplastic or polyolefin work piece.
A further object of the present invention is to provide a thermoplastic or polyolefin work piece having an appearance in-mold coating thereon, which has paint-like properties, such as high gloss, hardness, good adhesion and good weatherability.
A further object of the present invention is to provide a thermoplastic or polyolefin work piece having an in-mold coating thereon, which has good flow and coverage during molding, good adhesion, uniform color, durability, weather resistance, good surface qualities, and good paintability.